As Lux, there's no point in building MR vs Diana, right?

athenes should be your choice if you want defense. If you are still having trouble than health items are the way to go.

As if Assassins cared about resistances on squishies.

Was Void Staff the last item she built?

If so, MR rush would be great. Heavily counters the Flat Reduction and Penetration that Abyssal and Shoes give.

maybe u need poke frist . or dodge diana a q . u need zhonya when diana use R jumping to u, then just flash q a e R a W a .maybe she ll zhonya too when u ult . so maybe u keep ult for next q

When I play squishy long ranged champs like lux or AP kog (my experience is mostly with the latter), I tend to favor movespeed(/pinks) over resistances and try to play the don't get caught in the first place game, since even a 'tanky' lux is still going to be one of the lowest health champs in the game prone to quick death.

I'd consider chalice over morellos/fqc if I was worried about being flash->instaburst, although I often just run exhaust if I see I'm against a bursty assassin (instead of ignite which I tend to not be close enough to use more often than not).

Disclaimer: I am not a mid lane main.